Metal-Organic Frameworks - Omar Yaghi`s Laboratory

Transcription

The Chemistry and Applications of Metal-Organic Frameworks
Hiroyasu Furukawa et al.
Science 341, (2013);
DOI: 10.1126/science.1230444
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REVIEW SUMMARY
The Chemistry and Applications
of Metal-Organic Frameworks
READ THE FULL ARTICLE ONLINE
http://dx.doi.org/10.1126/science.1230444
Cite this article as H. Furukawa et al.,
Science 341, 1230444 (2013).
DOI: 10.1126/science.1230444
Hiroyasu Furukawa, Kyle E. Cordova, Michael O’Keeffe, Omar M. Yaghi*
Background: Metal-organic frameworks (MOFs) are made by linking inorganic and organic units
by strong bonds (reticular synthesis). The flexibility with which the constituents’ geometry, size, and
functionality can be varied has led to more than 20,000 different MOFs being reported and studied
within the past decade. The organic units are ditopic or polytopic organic carboxylates (and other
similar negatively charged molecules), which, when linked to metal-containing units, yield architecturally robust crystalline MOF structures with a typical porosity of greater than 50% of the MOF
crystal volume. The surface area values of such MOFs typically range from 1000 to 10,000 m2/g,
thus exceeding those of traditional porous materials such as zeolites and carbons. To date, MOFs
with permanent porosity are more extensive in their variety and multiplicity than any other class of
porous materials. These aspects have made MOFs ideal candidates for storage of fuels (hydrogen
and methane), capture of carbon dioxide, and catalysis applications, to mention a few.
ARTICLE OUTLINE
Advances: The ability to vary the size and nature of MOF structures without changing their underlying topology gave rise to the isoreticular principle and its application in making MOFs with the
largest pore aperture (98 Å) and lowest density (0.13 g/cm3). This has allowed for the selective
inclusion of large molecules (e.g., vitamin B12) and proteins (e.g., green fluorescent protein) and
the exploitation of the pores as reaction vessels. Along these lines, the thermal and chemical stability of many MOFs has made them amenable to postsynthetic covalent organic and metal-complex
functionalization. These capabilities enable substantial enhancement of gas storage in MOFs and
have led to their extensive study in the catalysis of organic reactions, activation of small molecules
(hydrogen, methane, and water), gas separation, biomedical imaging, and proton, electron, and ion
conduction. At present, methods are being developed for making nanocrystals and supercrystals of
MOFs for their incorporation into devices.
Gas Adsorption for Alternative Fuels and
Separations for Clean Air
Expansion of Structures by a Factor of 2 to 17
Exceptionally Large Pore Apertures
High Thermal and Chemical Stability
Postsynthetic Modification (PSM):
Crystals as Molecules
Catalytic Transformations Within the Pores
Proton Conductivity for Fuel Cell Applications
MOF Nanocrystals
The Materials Beyond
SUPPLEMENTARY MATERIALS
Materials and Methods
Figs. S1 to S8
Tables S1 to S5
References (135–363)
Related Web sites
Cambridge Structural Database reference codes
for MOFs
Metal-organic framework (MOF)
structures are amenable to expansion and incorporation of multiple
functional groups within their interiors. (A) The isoreticular expansion of
MOFs maintains the network’s topology
by using an expanded version of the
parent organic linker. Examples of
catalysis in MOFs are shown in the large
space created by IRMOF-74-XI; Me is a
methyl group. (B) Conceptual illustration of a multivariate MOF (MTV-MOF)
whose pores are decorated by heterogeneous mixtures of functionalities that
arrange in specific sequences. (Background) Optical image of zeolitic imidazolate framework (ZIF) crystals.
The list of author affiliations is available in the full article online.
*Corresponding author. E-mail: [email protected]
974
30 AUGUST 2013 VOL 341 SCIENCE www.sciencemag.org
Published by AAAS
Outlook: The precise control over the assembly of MOFs is expected to propel this field further into
new realms of synthetic chemistry in which far more sophisticated materials may be accessed. For
example, materials can be envisaged as having (i) compartments linked together to operate separately, yet function synergistically; (ii) dexterity to carry out parallel operations; (iii) ability to count,
sort, and code information; and (iv) capability of dynamics with high fidelity. Efforts in this direction
are already being undertaken through the introduction of a large number of different functional
groups within the pores of MOFs. This yields multivariate frameworks in which the varying arrangement of functionalities gives rise to materials that offer a synergistic combination of properties.
Future work will involve the assembly of chemical structures from many different types of building
unit, such that the structures’ function is dictated by the heterogeneity of the specific arrangement
of their constituents.
Design of Ultrahigh Porosity
er materials and because they lead to the previously elusive synthesis of solids by design. Unlike
other extended solids, MOFs maintain their underlying structure and crystalline order upon expansion of organic linkers and inorganic SBUs, as
well as after chemical functionalization, which
greatly widens the scope of this chemistry. We
review key developments in these areas and discuss the impact of this chemistry on applications
such as gas adsorption and storage, catalysis, and
proton conduction. We also discuss the concept
of MTV-MOFs in relation to the sequence of functionality arrangement that is influenced by the
electronic and/or steric interactions among the
functionalities. Highly functional synthetic crystalline materials can result from the use of such
techniques to create heterogeneity within MOF
structures.
The Chemistry and Applications of
Metal-Organic Frameworks
Hiroyasu Furukawa,1,2 Kyle E. Cordova,1,2 Michael O’Keeffe,3,4 Omar M. Yaghi1,2,4*
Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates
strong bonds between inorganic and organic units. Careful selection of MOF constituents can
yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics
allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and
catalysis, among other applications. The precision commonly exercised in their chemical
modification and the ability to expand their metrics without changing the underlying topology
have not been achieved with other solids. MOFs whose chemical composition and shape of
building units can be multiply varied within a particular structure already exist and may lead to
materials that offer a synergistic combination of properties.
*Corresponding author. E-mail: [email protected]
Total (CSD)
Extended (1D, 2D, 3D)
MOFs (3D)
12
ln(No. of structures)
6000
5000
4000
10
Doubling time
9.3 years
8
6
5.7 years
4
3.9 years
2
3000
0
1975 1980 1985 1990 1995 2000 2005 2010
Year
2000
1000
2010
2008
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
0
1974
1
Department of Chemistry, University of California, Berkeley,
CA 94720, USA. 2Materials Sciences Division, Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA. 3Department of
Chemistry, Arizona State University, Tempe, AZ 87240, USA.
4
NanoCentury KAIST Institute and Graduate School of Energy,
Environment, Water, and Sustainability (World Class University), Daejeon 305-701, Republic of Korea.
7000
1972
T
Design of Ultrahigh Porosity
During the past century, extensive work was done
on crystalline extended structures in which metal
ions are joined by organic linkers containing Lewis
base–binding atoms such as nitriles and bipyridines
(8, 9). Although these are extended crystal structures and not large discrete molecules such as polymers, they were dubbed coordination “polymers”—
a term that is still in use today, although we prefer
the more descriptive term MOFs, introduced in
1995 (10) and now widely accepted. Because
these structures were constructed from long organic linkers, they encompassed void space and
therefore were viewed to have the potential to be
of MOFs with ultrahigh porosity and unusually
large pore openings (5). (iii) Postsynthetic modification (PSM) of MOFs—incorporating organic
units and metal-organic complexes through reactions with linkers—has emerged as a powerful
tool for changing the reactivity of the pores (e.g.,
creating catalytic sites) (6). (iv) Multivariate MOFs
(MTV-MOFs), in which multiple organic functionalities are incorporated within a single framework,
have provided many opportunities for designing
complexity within the pores of MOFs in a controlled manner (7).
Below, we focus on these aspects of MOF
chemistry because they are rarely achieved in oth-
No. of MOF structures
he past decade has seen explosive growth
in the preparation, characterization, and
study of materials known as metal-organic
frameworks (MOFs). These materials are constructed by joining metal-containing units [secondary building units (SBUs)] with organic linkers,
using strong bonds (reticular synthesis) to create
open crystalline frameworks with permanent porosity (1). The flexibility with which the metal
SBUs and organic linkers can be varied has led
to thousands of compounds being prepared and
studied each year (Figs. 1 and 2). MOFs have exceptional porosity and a wide range of potential
uses including gas storage, separations, and catalysis (2). In particular, applications in energy
technologies such as fuel cells, supercapacitors,
and catalytic conversions have made them objects of extensive study, industrial-scale production, and application (2–4).
Among the many developments made in this
field, four were particularly important in advancing the chemistry of MOFs: (i) The geometric
principle of construction was realized by the linking of SBUs with rigid shapes such as squares
and octahedra, rather than the simpler node-andspacer construction of earlier coordination networks in which single atoms were linked by ditopic
coordinating linkers (1). The SBU approach not
only led to the identification of a small number
of preferred (“default”) topologies that could be
targeted in designed syntheses, but also was central to the achievement of permanent porosity in
MOFs (1). (ii) As a natural outcome of the use of
SBUs, a large body of work was subsequently
reported on the use of the isoreticular principle
(varying the size and nature of a structure without
changing its underlying topology) in the design
Year
Fig. 1. Metal-organic framework structures (1D, 2D, and 3D) reported in the Cambridge Structural Database (CSD) from 1971 to 2011. The trend shows a striking increase during this period for
all structure types. In particular, the doubling time for the number of 3D MOFs (inset) is the highest
among all reported metal-organic structures.
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SCIENCE
VOL 341
30 AUGUST 2013
1230444-1
REVIEW
REVIEW
A
M2(CO2)4
(M = Cu, Zn, Fe,
Mo, Cr, Co, and
Ru)
M3O(CO2)6
(M = Zn, Cr,
In, and Ga)
Zn4O(CO2)6
Zr6O4(OH)4(CO2)12
Cu2(CNS)4
Zn(C3H3N2)4
M3O3(CO2)3
(M = Zn, Mg,
Co, Ni, Mn,
Fe, and Cu)
Ni4(C3H3N2)8
Al(OH)(CO2)2
VO(CO2)2
B
COOH
COOH
X
COOH
COOH
O
COOH
OH
COOH
X
HO
OH
O
COOH
COOH
Oxalic acid
Fumaric
acid
H2BDC
X
COOH
HOOC
COOH
H4DOT
H2BDC-X
(X = Br, OH,
NO2, and NH2)
COOH
COOH
COOH
X
H3BTC
H2BDC-(X)2
(X = Me, Cl,
COOH, OC3H5,
and OC7H7)
X
H3BTE (X = C≡C)
H3BBC (X = C6H4)
X
HOOC
COOH
HN N
COOH
OH
HO3S
HOOC
HOOC
COOH
HO
H4ATC
SO3H
N
OH
SO3H
HOOC
N
HOOC
NH2
H2N
NH
N
X
O
COOH
OH
N
HOOC
N
COOH
N N
COOH
COOH
HOOC
N
COOH
OH
ADP
TIPA
H4ADB
COOH
COOH
HOOC
COOH
COOH
COOH
Gly-Ala
N
HOOC
X
H3BTB (X = CH)
H3TATB (X = N)
DTOA
HOOC
H
N
COOH
X
S
H3BTP
H2N
N
COOH
N
HN
COOH
H3ImDC
H3THBTS
N
HOOC
S
NH
X
X
COOH
H6BTETCA
COOH
COOH
N
X
COOH
Cl
COOH
HOOC
COOH
OH
OH
Cl
H6TPBTM (X = CONH)
H6BTEI (X = C≡C)
H6BTPI (X = C6H4)
N
H6BHEI (X = C≡C−C≡C)
DCDPBN
H6BTTI (X = (C6H4)2)
H6PTEI (X = C6H4−C≡C)
H6TTEI (X = C≡C-C6H4-C≡C)
H6BNETPI (X = C≡C−C6H4−C≡C−C≡C)
H6BHEHPI (X = (C6H4−C≡C)2)
O
O
O
O
+
O
N
N
Ir
O
O
O
O
N
O
N
COOH
BPP34C10DA
COOH
Ir(H2DPBPyDC)(PPy)2+
HO
HO
COOH
H4DH9PhDC
COOH
H4DH11PhDC
Fig. 2. Inorganic secondary building units (A) and organic linkers (B) referred to in the text. Color code: black, C; red, O; green, N; yellow, S; purple,
P; light green, Cl; blue polyhedra, metal ions. Hydrogen atoms are omitted for clarity. AIPA, tris(4-(1H-imidazol-1-yl)phenyl)amine; ADP, adipic acid; TTFTB4–,
4,4′,4′′,4′′′-([2,2′-bis(1,3-dithiolylidene)]-4,4′,5,5′-tetrayl)tetrabenzoate.
1230444-2
30 AUGUST 2013
VOL 341
SCIENCE
www.sciencemag.org
Na(OH)2(SO3)3
In(C5HO4N2)4
HO
Zr6O8(CO2)8
permanently porous, as is the case for zeolites. The
porosity of these compounds was investigated in
the 1990s by forcing gas molecules into the crevices at high pressure (11). However, proof of permanent porosity requires measurement of reversible
gas sorption isotherms at low pressures and temperatures. Nonetheless, as we remarked at that
time (12), it was then commonplace to refer to
materials as “porous” and “open framework” even
though such proof was lacking. The first proof of
permanent porosity of MOFs was obtained by measurement of nitrogen and carbon dioxide isotherms
on layered zinc terephthalate MOF (12).
A major advance in the chemistry of MOFs
came in 1999 when the synthesis, x-ray singlecrystal structure determination, and low-temperature,
low-pressure gas sorption properties were reported
for the first robust and highly porous MOF, MOF-5
(13). This archetype solid comprises Zn4O(CO2)6
octahedral SBUs each linked by six chelating
1,4-benzenedicarboxylate (BDC2–) units to give
a cubic framework (Fig. 2, figs. S2 and S3, and
tables S1 and S2). The architectural robustness of
MOF-5 allowed for gas sorption measurements,
which revealed 61% porosity and a BrunauerEmmett-Teller (BET) surface area of 2320 m2/g
(2900 m2/g Langmuir). These values are substantially higher than those commonly found for zeolites and activated carbon (14).
To prepare MOFs with even higher surface
area (ultrahigh porosity) requires an increase in
storage space per weight of the material. Longer
organic linkers provide larger storage space and a
greater number of adsorption sites within a given
material. However, the large space within the crystal framework makes it prone to form interpenetrating structures (two or more frameworks grow
and mutually intertwine together). The most effective way to prevent interpenetration is by making
MOFs whose topology inhibits interpenetration
because it would require the second framework
to have a different topology (15). Additionally,
it is important to keep the pore diameter in the
micropore range (below 2 nm) by judicious selection of organic linkers in order to maximize
the BET surface area of the framework, because it
is known that BET surface areas obtained from
isotherms are similar to the geometric surface areas
derived from the crystal structure (16). In 2004,
MOF-177 [Zn4O(BTB)2; BTB = 4,4′,4′′-benzene1,3,5-triyl-tribenzoate] was reported with the highest surface area at that time (BET surface area =
3780 m2/g, porosity = 83%; Figs. 3A and 4) (15),
which satisfies the above requirements. In 2010,
the surface area was doubled by MOF-200 and
MOF-210 [Zn4O(BBC)2 and (Zn4O)3(BTE)4(BPDC)3,
respectively; BBC3– = 4,4′,4′′-(benzene-1,3,5triyl-tris(benzene-4,1-diyl))tribenzoate; BTE =
4,4′,4′′-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))
tribenzoate; BPDC = biphenyl-4,4′-dicarboxylate]
to produce ultrahigh surface areas (4530 m2/g
and 6240 m2/g, respectively) and porosities (90%
and 89%) (17).
An x-ray diffraction study performed on a single crystal of MOF-5 dosed with nitrogen or argon
gas identified the adsorption sites within the pores
(18). The zinc oxide SBU, the faces, and, surprisingly, the edges of the BDC2– linker serve as
adsorption sites. This study uncovered the origin
of the high porosity and has enabled the design of
MOFs with even higher porosities (Fig. 4 and table S3). Moreover, it has been reported that expanded tritopic linkers based on alkyne rather than
phenylene units should increase the number of
adsorption sites and increase the surface area (19).
NU-110 [Cu3(BHEHPI); BHEHPI6– = 5,5′,5′′((((benzene-1,3,5-triyltris(benzene-4,1-diyl))
tris(ethyne-2,1-diyl))-tris(benzene-4,1-diyl))
tris(ethyne-2,1-diyl))triisophthalate], whose organic
linker is replete with such edges, displayed a surface area of 7140 m2/g (Table 1) (7, 17, 20–32).
For many practical purposes, such as storing
gases, calculating the surface area per volume
is more relevant. By this standard, the value for
MOF-5, 2200 m2/cm3, is among the very best
reported for MOFs (for comparison, the value for
NU-110 is 1600 m2/cm3). Note that the external
surface area of a nanocube with edges measuring
3 nm would be 2000 m2/cm3. However, nanocrystallites on this scale with “clean” surfaces
would immediately aggregate, ultimately leaving
their potential high surface area inaccessible.
Expansion of Structures by a Factor of 2 to 17
A family of 16 cubic MOFs—IRMOF-1 [also
known as MOF-5, which is the parent MOF of
the isoreticular (IR) series] to IRMOF-16—with
the same underlying topology (isoreticular) was
made with expanded and variously functionalized
organic linkers (figs. S2 and S3) (1, 5). This development heralded the potential for expanding
and functionalizing MOFs for applications in gas
storage and separations. The same work demonstrated that a large number of topologically identical but functionally distinctive structures can be
made. Note that the topology of these isoreticular
MOFs is typically represented with a three-letter
code, pcu, which refers to its primitive cubic net
(33). One of the smallest isoreticular structures
of MOF-5 is Zn4O(fumarate)3 (34); one of the
largest is IRMOF-16 [Zn4O(TPDC)3; TPDC2– =
terphenyl-4,4′′-dicarboxylate] (5) (fig. S2). In this
expansion, the unit cell edge is doubled and its
volume is increased by a factor of 8. The degree
of interpenetration, and thus the porosity and density of these materials, can be controlled by changing the concentration of reactants, temperature, or
other experimental conditions (5).
The concept of the isoreticular expansion is
not simply limited to cubic (pcu) structures, as
illustrated by the expansion of MOF-177 to give
MOF-180 [Zn4O(BTE)2] and MOF-200, which
use larger triangular organic linkers (qom net; Fig.
3A and fig. S4) (15, 17). Contrary to the MOF-5
type of expanded framework, expanded structures
of MOF-177 are noninterpenetrating despite the
high porosity of these MOFs (89% and 90% for
MOF-180 and MOF-200, respectively). These
results highlight the critical role of selecting
topology.
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SCIENCE
VOL 341
Another MOF of interest is known as HKUST-1
[Cu3(BTC)2; BTC3– = benzene-1,3,5-tricarboxylate]
(35); it is composed of Cu paddlewheel [Cu2(CO2)4]
SBUs (Fig. 2A) and a tritopic organic linker, BTC3–.
Several isoreticular structures have been made
by expansion with TATB3– [4,4′,4′′-(1,3,5-triazine2,4,6-triyl)tribenzoate], TATAB3– [4,4′,4′′-((1,3,5triazine-2,4,6-triyl)tris(azanediyl))tribenzoate],
TTCA3– [triphenylene-2,6,10-tricarboxylate],
HTB3– [4,4′,4′′-(1,3,3a1,4,6,7,9-heptaazaphenalene2,5,8-triyl)tribenzoate], and BBC3– linkers (tbo
net; Fig. 3B, fig. S1 and S5, and tables S1 and
S2) (21, 36–39). The cell volume for the largest
reported member [MOF-399, Cu3(BBC)2] is
17.4 times that of HKUST-1. MOF-399 has the
highest void fraction (94%) and lowest density
(0.126 g/cm3) of any MOF reported to date (21).
Cu paddlewheel units are also combined with
various lengths of hexatopic linkers to form another isoreticular series. The first example of one
of these MOFs is Zn3(TPBTM) [TPBTM6– =
5,5′,5′′-((benzene-1,3,5-tricarbonyl)tris(azanediyl))
triisophthalate], which has a ntt net (40). Shortly
after this report, several isoreticular MOF structures were synthesized (Fig. 3C and fig. S6)
(19, 20, 24, 41–48): Cu3(TPBTM), Cu3(TDPAT),
NOTT-112 [Cu3(BTPI)], NOTT-116 [also known
as PCN-68; Cu3(PTEI)], PCN-61 [Cu3(BTEI)],
PCN-66 [Cu3(NTEI)], PCN-69 [also known as
NOTT-119; Cu3(BTTI)], PCN-610 [also known
as NU-100; Cu3(TTEI)], NU-108 [Cu3(BTETCA)],
NU-109 [Cu3(BNETPI)], NU-110, and NU-111
[Cu3(BHEI)] TDPAT6– = 5,5′,5′′-((1,3,5-triazine2,4,6-triyl)tris(azanediyl))triisophthalate; BTPI6– =
5,5′,5′′-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))
triisophthalate; PTEI6– = 5,5′,5′′-((benzene-1,3,5triyl-tris(benzene-4,1-diyl))tris(ethyne-2,1-diyl))
triisophthalate; BTEI6– = 5,5′,5′′-(benzene-1,3,5triyl-tris(ethyne-2,1-diyl))triisophthalate; NTEI6– =
5,5′,5′′-((nitrilotris(benzene-4,1-diyl))tris(ethyne2,1-diyl))triisophthalate; BTTI6– = 5,5′,5′′(benzene-1,3,5-triyl-tris(biphenyl-4,4′-diyl))
triisophthalate; TTEI6– = 5,5′,5′′-(((benzene1,3,5-triyl-tris(ethyne-2,1-diyl))tris(benzene-4,1diyl))tris(ethyne-2,1-diyl))triisophthalate;
BTETCA6– = 5′,5′′′′,5′′′′′′′-(benzene-1,3,5-triyl-tris
(ethyne-2,1-diyl))tris(([1,1′:3′,1′′-terphenyl]-4,4′′dicarboxylate)); BNETPI6– = 5,5′,5″-(((benzene1,3,5-triyl-tris(ethyne-2,1-diyl))tris(benzene-4,1-diyl))
tris(buta-1,3-diyne-4,1-diyl))triisophthalate;
BHEI6– = 5,5′,5″-(benzene-1,3,5-triyl-tris(buta1,3-diyne-4,1-diyl))triisophthalate]. Isoreticular
materials are not necessarily expansions of the
original parent MOF, as exemplified by NU-108,
because the ntt family has a linker (BTETCA6–)
with two branching points and two kinds of links
(figs. S1B and S7).
A wide variety of metal ions form metalcarboxylate units, and isostructural MOFs can be
synthesized by replacing the metal ions in the
inorganic SBUs. Indeed, after the appearance of
HKUST-1 [Cu3(BTC)2], an isostructural series of
HKUST-1 [M3(BTC)2, where M = Zn(II), Fe(II),
Mo(II), Cr(II), Ru(II)] was prepared by several groups (fig. S5) (49–53). In the same way as
30 AUGUST 2013
1230444-3
REVIEW
REVIEW
A
B
Tritopic linker
Tritopic linker
Zn4O(CO2)6
Cu2(CO2)4
× 5.5
× 1.8
Cu3(BTC)2, HKUST-1
MOF-199 (tbo)
Zn4O(BTE)2
MOF-180 (qom)
× 17.4
× 2.7
Zn4O(BBC)2
MOF-200 (qom)
20 Å
Hexatopic linker
C
20 Å
Cu3(BBC)2,
MOF-399 (tbo)
D
Cu2(CO2)4
× 6.5
Mg3O3(CO2)3
Mg2(DOT), Mg-MOF-74
(IRMOF-74-I) (etb)
Tetratopic
linker
Cu3(TDPAT) (ntt)
Mg2(DH5PhDC),
IRMOF-74-V (etb)
× 16.1
× 2.2
Cu3(NTEI),
PCN-66 (ntt)
× 6.0
30 Å
Cu3(BHEHPI),
NU-110 (ntt)
30 Å
Fig. 3. Isoreticular expansion of metal-organic frameworks with
qom, tbo, ntt, and etb nets. (A to D) The isoreticular (maintaining same
topology) expansion of archetypical metal-organic frameworks resulting
from discrete [(A), (B), and (C)] and rod inorganic SBUs (D) combined with
tri-, hexa-, and tetratopic organic linkers to obtain MOFs in qom (A), tbo
(B), ntt (C), and etb (D) nets, respectively. Each panel shows a scaled
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Mg2(DH11PhDC),
IRMOF-74-XI (etb)
comparison of the smallest, medium, and largest crystalline structures of
MOFs representative of these nets. The large yellow and green spheres
represent the largest sphere that would occupy the cavity. Numbers above
each arrow represent the degree of volume expansion from the smallest
framework. Color code is same as in Fig. 2; hydrogen atoms are omitted for
clarity.
SCIENCE
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Zn4O(BTB)2
MOF-177 (qom)
Cu3(TATB)2, PCN-6’ (tbo)
discrete inorganic SBUs, the infinite inorganic
rod-type SBUs were also used to synthesize
isostructural MOF-74 [Zn2(DOT); DOT =
dioxidoterephthalate] (54) using divalent metal
ions such as Mg, Co, Ni, and Mn (fig. S8) (55).
Exceptionally Large Pore Apertures
Pore openings of MOFs are typically large enough
(up to 2 nm) to accommodate small molecules,
but rarely are they of appropriate size to permit
inclusion of large molecules such as proteins.
The best way to increase pore apertures is to use
infinite rod-shaped SBUs with linkers of arbitrary
length providing periodicity in the other two dimensions, which does not allow for interpenetrating structures. This strategy was implemented
by expanding the original phenylene unit of MOF-74
[M2(DOT); M2+ = Zn, Mg] structure (54) to 2, 3,
8000
Typical
conventional
porous
materials
7000
MOF-210 (3.60)
NU-100 (2.82)
MOF-177 (2.00)
UMCM-2
(2.32)
IRMOF-20 (1.53)
MOF-5 (1.48)
5000
4000
DUT-49
(2.91)
MIL-101
MOF-5 (2.15)
(1.56)
MIL-101 (2.00)
MOF-177 (1.59)
1999
2004
2010
Bio-MOF-100 (4.30)
2008
DUT-23(Co) (2.03)
2006
MOF-200 (3.59)
NOTT-112 (1.59)
UMCM-1 (2.24)
0
PCN-6 (1.41)
1000
MOF-5
(1.04)
MIL-100 (1.10)
2000
Silicas (1.15)
3000
Carbons (0.60)
MOF-5 (1.20)
Zeolites (0.30)
BET surface area (m2/g)
6000
NU-110
(4.40)
NOTT-119 (2.35)
MOFs
2012
Year
Fig. 4. Progress in the synthesis of ultrahigh-porosity MOFs. BET surface areas of MOFs and typical
conventional materials were estimated from gas adsorption measurements. The values in parentheses
represent the pore volume (cm3/g) of these materials.
4, 5, 6, 7, 9, and 11 phenylene units [DH2PhDC4–
to DH11PhDC4–, respectively; Fig. 2B, Fig. 3D, and
figs. S1B and S8] (22). Crystal structures revealed
that pore apertures for this series of MOF-74 structures (termed IRMOF-74-I to IRMOF-74-XI) ranged
from 14 to 98 Å. The presence of the large pore
apertures was also confirmed by transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) observation as well as argon
adsorption measurements of the guest-free materials. As expected, the pore aperture of IRMOF-74IX is of sufficient size to allow for green fluorescent
protein (barrel structure with diameter of 34 Å
and length of 45 Å) to pass into the pores without
unfolding. More important, the large pore aperture
is of benefit to the surface modification of the pores
with various functionalities without sacrificing the
porosity (22). An oligoethylene glycol–functionalized
IRMOF-74-VII [Mg2(DH7PhDC-oeg)] allows inclusion of myoglobin, whereas IRMOF-74-VII
with hydrophobic hexyl chains showed a negligible amount of inclusion.
High Thermal and Chemical Stability
Because MOFs are composed entirely of strong
bonds (e.g., C-C, C-H, C-O, and M-O), they show
high thermal stability ranging from 250° to 500°C
(5, 56–58). It has been a challenge to make chemically stable MOFs because of their susceptibility
to link-displacement reactions when treated with
solvents over extended periods of time (days). The
first example of a MOF with exceptional chemical
stability is zeolitic imidazolate framework–8 [ZIF8, Zn(MIm)2; MIm– = 2-methylimidazolate], which
was reported in 2006 (56). ZIF-8 is unaltered after
immersion in boiling methanol, benzene, and water
for up to 7 days, and in concentrated sodium
hydroxide at 100°C for 24 hours.
Table 1. Typical properties and applications of metal-organic frameworks. Metal-organic frameworks exhibiting the lowest and highest values
for the indicated property, and those reported first for selected applications, are shown.
Property or application
Lowest reported value
Density
Highest reported value
Pore aperture
Number of organic linkers
Degrees of interpenetration
BET surface area
Pore volume
Excess hydrogen uptake (77 K, 56 bar)
Excess methane uptake (290 K, 35 bar)
Excess carbon dioxide uptake (298 K, 50 bar)
Proton conductivity (98% relative humidity, 25°C)
Charge mobility
Lithium storage capacity (after 60 cycles)
Earliest report
Catalysis by a MOF
Gas adsorption isotherm and permanent porosity
Asymmetric catalysis with a homochiral MOF
Production of open metal site
PSM on the organic linker
Use of a MOF for magnetic resonance imaging
Compound
Achieved value or year of report
Reference
MOF-399
0.126 g/cm3
(21)
IRMOF-74-XI
MTV-MOF-5
Ag6(OH)2(H2O)4(TIPA)5
NU-110
NU-110
NU-100
PCN-14
MOF-200
(NH4)2(ADP)[Zn2(oxalate)3]·3H2O
Zn2(TTFTB)
Zn3(HCOO)6
98 Å
8
54
7140 m2/g
4.40 cm3/g
9.0 wt%
212 mg/g
2347 mg/g
8 × 10−3 S/cm
0.2 cm2/V·s
560 mAh/g
(22)
(7)
(23)
(20)
(20)
(24)
(25)
(17)
(26)
(27)
(28)
Cd(BPy)2(NO3)2
MOF-2
POST-1
MOF-11
POST-1
MOF-73
1994
1998
2000
2000
2000
2008
(29)
(12)
(31)
(30)
(31)
(32)
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MOFs based on the Zr(IV) cuboctahedral SBU
(Fig. 2A) also show high chemical stability;
UiO-66 [Zr6O4(OH)4(BDC)6] and its NO2- and
Br-functionalized derivatives demonstrated high
acid (HCl, pH = 1) and base resistance (NaOH,
pH = 14) (57, 58). The stability also remains
when tetratopic organic linkers are used; both
MOF-525 [Zr6O4(OH)4(TpCPP-H2)3; TpCPP =
tetra-para-carboxyphenylporphyrin] and 545
[Zr6O8(TpCPP-H2)2] are chemically stable in
methanol, water, and acidic conditions for 12 hours
(59). Furthermore, a pyrazolate-bridged MOF
[Ni3(BTP)2; BTP3– = 4,4′,4″-(benzene-1,3,5triyl)tris(pyrazol-1-ide)] is stable for 2 weeks in a
wide range of aqueous solutions (pH = 2 to 14) at
100°C (60). The high chemical stability observed
in these MOFs is expected to enhance their performance in the capture of carbon dioxide from
humid flue gas and extend MOFs’ applications to
water-containing processes.
Postsynthetic Modification (PSM):
Crystals as Molecules
The first very simple, but far from trivial, example
of PSM was with the Cu paddlewheel carboxylate
MOF-11 [Cu2(ATC); ATC4– = adamantane-1,3,5,7tetracarboxylate] (30). As-prepared Cu atoms are
bonded to four carboxylate O atoms, and the coordination shell is completed typically with coordinated water (Fig. 2A). Subsequent removal of
the water from the immobilized Cu atom leaves a
coordinatively unsaturated site (“open metal site”).
Many other MOFs with such sites have now been
generated and have proved to be exceptionally
favorable for selective gas uptake and catalysis
(61–63).
The first demonstration of PSM on the organic link of a MOF was reported in 2000 for a
homochiral MOF, POST-1 [Zn3(m3-O)(D-PTT)6;
D-PTT – = (4S,5S )-2,2-dimethyl-5-(pyridin-4ylcarbamoyl)-1,3-dioxolane-4-carboxylate] (31).
It involved N-alkylation of dangling pyridyl functionalities with iodomethane and 1-iodohexane to
produce N-alkylated pyridinium ions exposed to
the pore cavity.
More recently, PSM was applied to the dangling amine group of IRMOF-3 [Zn4O(BDC-NH2)3]
crystals (6). The MOF was submerged in a
dichloromethane solution containing acetic anhydride to give the amide derivative in >80% yield.
Since then, a large library of organic reactions
have been used to covalently functionalize MOF
backbones (table S4) (64, 65).
UMCM-1-NH2 [(Zn4O)3(BDC-NH2)3(BTB)4]
was also acylated with benzoic anhydride to
produce the corresponding amide functionality
within the pores (66). The structures of both
IRMOF-3 and UMCM-1-NH2 after modification
showed increased hydrogen uptake relative to the
parent MOFs, even though there was a reduction
in overall surface area (66). PSM has also been
used to dangle catalytically active centers within
the pores. In an example reported in 2005, a Cd-based
MOF built from 6,6′-dichloro-4,4′-di(pyridin-4-yl)[1,1′-binaphthalene]-2,2′-diol (DCDPBN),
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[CdCl2(DCDPBN)], used orthogonal dihydroxy
functionalities to coordinate titanium isopropoxide
[Ti(OiPr)4], thus yielding a highly active, enantioselective asymmetric Lewis acid catalyst (67).
UMCM-1-NH2 was also functionalized in such a
manner to incorporate salicylate chelating groups,
which were subsequently metallated with Fe(III)
and used as a catalyst for Mukaiyama aldol reactions over multiple catalytic cycles without loss of
activity or crystallinity (68). Indeed, the remarkable
retention of MOF crystallinity and porosity after
undergoing the transformation reactions clearly demonstrates the use of MOF crystals as molecules (69).
Catalytic Transformations Within the Pores
The high surface areas, tunable pore metrics, and
high density of active sites within the very open
structures of MOFs offer many advantages to
their use in catalysis (table S5). MOFs can be
used to support homogeneous catalysts, stabilize short-lived catalysts, perform size selectivity, and encapsulate catalysts within their pores
(70). The first example of catalysis in an extended framework, reported in 1994, involved the
cyanosilylation of aldehydes in a Cd-based framework [Cd(BPy)2(NO3)2; BPy = 4,4′-bipyridine]
as a result of axial ligand removal (29). This
study also highlighted the benefits of MOFs as
size-selective catalysts by excluding large substrates from the pores.
In 2006, it was shown that removal of solvent
from HKUST-1 exposes open metal sites that
may act as Lewis acid catalysts (71). MIL-101
[Cr3X(H2O)2O(BDC)3; X = F, OH] and Mn-BTT
{Mn3[(Mn4Cl)3(BTT)8]2; BTT3– = 5,5′,5″-(benzene1,3,5-triyl)tris(tetrazol-2-ide)} have also been identified as Lewis acid catalysts in which the metal
oxide unit functions as the catalytic site upon ligand removal (62, 72). In addition, alkane oxidation, alkene oxidation, and oxidative coupling
reactions have also been reported; they all rely on
the metal sites within the SBUs for catalytic activity (73–75). The study of methane oxidation
in vanadium-based MOF-48 {VO[BDC-(Me)2];
Me = methyl} is promising because the catalytic
turnover and yield for this oxidation far exceed
those of the analogous homogeneous catalysts (73).
One early example of the use of a MOF as a heterogeneous catalyst is PIZA-3 [Mn2(TpCPP)2Mn3],
which contains a metalloporphyrin as part of
the framework (76). PIZA-3 is capable of hydroxylating alkanes and catalyzes the epoxidation of
olefins. Schiff-base and binaphthyl metal complexes
have also been incorporated into MOFs to achieve
olefin epoxidation and diethyl zinc (ZnEt2) additions to aromatic aldehydes, respectively (67, 77).
The incorporation of porphyrin units within
the pores of MOFs can be accomplished during
the synthesis (a “ship-in-a-bottle” approach that captures the units as the pores form), as illustrated for
the zeolite-like MOF rho-ZMOF [In(HImDC)2·X;
HImDC2– = imidazoledicarboxylate, X– = counteranion] (78). The pores of this framework accommodate high porphyrin loadings, and the pore
aperture is small enough to prevent porphyrin from
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leaching out of the MOF. The porphyrin metal sites
were subsequently metallated and used for the
oxidation of cyclohexane. The same approach has
been applied to several other systems in which
polyoxometalates are encapsulated within MIL101(Cr) and HKUST-1 for applications in the
oxidation of alkenes and the hydrolysis of esters
in excess water (79, 80).
Integration of nanoparticles for catalysis by
PSM has been carried out to enhance particle
stability or to produce uniform size distributions.
Palladium nanoparticles were incorporated within MIL-101(Cr) for cross-coupling reactions
(81, 82). Most recently, a bifunctional catalytic
MOF {Zr6O4(OH)4[Ir(DPBPyDC)(PPy)2·X]6;
DPBPyDC2– = 4,4′-([2,2′-bipyridine]-5,5′diyl)dibenzoate, PPy = 2-phenylpyridine} capable
of water-splitting reactions was reported (83). This
MOF uses the organic linker and an encapsulated
nanoparticle to transfer an electron to a proton in
solution, leading to hydrogen evolution.
Gas Adsorption for Alternative Fuels and
Separations for Clean Air
Much attention is being paid to increasing the
storage of fuel gases such as hydrogen and methane under practical conditions. The first study of
hydrogen adsorption was reported in 2003 for
MOF-5 (84). This study confirmed the potential
of MOFs for application to hydrogen adsorption,
which has led to the reporting of hydrogen adsorption data for hundreds of MOFs (85). In general, the functionality of organic linkers has little
influence on hydrogen adsorption (86), whereas
increasing the pore volume and surface area of
MOFs markedly enhances the gravimetric hydrogen uptake at 77 K and high pressure, as exemplified by the low-density materials: NU-100 and
MOF-210 exhibit hydrogen adsorption as high as
7.9 to 9.0 weight percent (wt%) at 56 bar for both
MOFs and 15 wt% at 80 bar for MOF-210 (17, 24).
However, increasing the surface area is not always
an effective tool for increasing the volumetric
hydrogen adsorption, which can be accomplished
by increasing the adsorption enthalpy of hydrogen (Qst) (87). In this context, open metal sites
have been suggested and used to enhance the
hydrogen uptake capacity and to improve Qst
(61, 85). Two MOFs with this characteristic,
Zn3(BDC)3[Cu(Pyen)] [Pyen2– =5,5′-((1E,1′E)-(ethane1,2-diyl-bis(azanylylidene))bis(methanylylidene))bis(3methylpyridin-4-ol)] and Ni-MOF-74, have the
highest reported initial Qst values: 15.1 kJ/mol and
12.9 kJ/mol, respectively (58, 88). Metal impregnation has also been suggested by computation
as a method for increasing the Qst values (89).
Experiments along these lines show that doping MOFs with alkali metal cations yields only
modest enhancements in the total hydrogen uptake and Qst values (90, 91). Although some challenges remain in meeting the U.S. Department
of Energy (DOE) system targets (5.5 wt% and
40 g/liter at –40° to 60°C below 100 bar) for hydrogen adsorption (85), Mercedes-Benz has already deployed MOF hydrogen fuel tanks in a
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REVIEW
fuel cell–powered demonstration model, the
F125 (92).
An alternative high-density fuel source to hydrogen and gasoline is natural gas (methane). The
first study of high-pressure methane adsorption
in an extended metal-organic structure was reported in 2000 for CuSiF6(BPy)2 (93), which demonstrated an uptake capacity of 104 mg/g at 36 atm
and 298 K. As is the case for hydrogen adsorption,
the total gravimetric methane uptake capacity is
generally proportional to the pore volume of MOFs.
The calculated total uptake values for MOF-177,
MOF-200, and MOF-210 are 345 mg/g, 446 mg/g,
and 476 mg/g, respectively, at 80 bar and 298 K.
These values are much greater than those of any
other MOF (17). The amount of methane stored
in a vessel filled with one of these MOFs is at
least double the amount that could be stored in an
empty vessel at room temperature and pressures
up to 80 bar (17). This technology is now being
commercialized by BASF for automobile fueling
(94). The optimal material is not necessarily the
one with highest uptake, but rather one that allows minimal uptake at low pressure and highest
uptake at higher pressure so as to maximize the
working capacity.
MOFs also offer reversible carbon dioxide
adsorption and are promising materials for the
selective capture of carbon from the atmosphere
and flue gas. Carbon dioxide adsorption in MOFs
was first reported in 1998 for MOF-2 [Zn(BDC)]
(12). In 2005, a detailed study of carbon dioxide
adsorption in a series of MOFs at room temperature showed MOF-177 to have an uptake capacity of 1470 mg/g at 35 bar (95). This uptake of
carbon dioxide exceeded that of any known porous material under such conditions. The large
quadrupole moment of carbon dioxide molecules
causes them to interact with the framework more
strongly than hydrogen and methane. As expected,
the best excess carbon dioxide uptake reported
to date was observed in a MOF with ultrahigh
porosity, MOF-200 (2437 mg/g at 50 bar and
298 K) (17). In practical terms, a gas tank filled
with MOF-177 or MOF-200 would store 9 times
or 17 times as much carbon dioxide at 35 bar,
respectively, as the corresponding pressurized tank
without MOF. On the other hand, many carbon
dioxide capture applications will operate at low
pressure so that the Henry’s law constant (i.e.,
initial slope of the isotherm) can be used as an
indicator of the carbon dioxide selectivity. MOFs
with open metal sites were found to have desirable high initial Qst values of 62 kJ/mol and
47 kJ/mol for MIL-101(Cr) and Mg-MOF-74,
respectively (96, 97), thereby offering enhanced
carbon dioxide uptake and selectivity at low pressures. Basic nitrogen centers have also been postsynthetically added to MOFs with open metal sites
by using N,N′-dimethylethylenediamine (mmen),
where the highest initial Qst for H3[(Cu4Cl)3
(BTTri)8(mmen)12] [BTTri3– = 4,4′,4″-(benzene1,3,5-triyl)tris(1,2,3-triazol-1-ide)] is estimated to
be 96 kJ/mol (98). Because the ideal material for
carbon dioxide capture from flue and combustion
gases requires high selectivity in the presence of
water, it is useful to target MOFs in which the
competition between carbon dioxide and water
for adsorption is minimized. In this respect, chemical binding of carbon dioxide in a recent MOF to
make organic carbonates reversibly is a promising
approach (99).
Gas storage experiments on MOFs have also
been extended to the separation of hydrocarbons,
toxic molecules (e.g., ammonia and chlorine), and
water. For instance, Cu2(PZDC)2(Pyz) (PZDC =
pyrazine-2,3-dicarboxylate; Pyz = pyrazine) selectively takes up acetylene over carbon dioxide
through hydrogen bonding between acetylene and
oxygen atoms on the MOF internal surface (100).
Ammonia was previously considered to be too reactive for MOFs; however, chemically stable Zr-MOFs,
such as UiO-66-NH2 [Zr6O4(OH)4(BDC-NH2)6]
and other derivatives, maintain their structures after the process of ammonia adsorption and desorption (101).
Gas separation processes in MOFs generally
rely on both the size of the pores and the affinity
of MOFs for the targeted gases. After the discovery of permanent porosity of MOFs, equilibrium adsorption isotherms for various gases were
collected to estimate potential gas selectivity. High
selectivity calculated from equilibrium data does
not guarantee high selectivity under dynamic gas
separation conditions. The latter is desirable in industrial processes, because the diffusion rate and
Qst value of gas binding in the adsorbent are sensitive to the operating conditions. One of the earliest examples of a dynamic separation was performed
using a gas chromatographic column filled with
MOF-508 [Zn2(BDC)2(BPy)] to separate alkanes
such as n-pentane, n-hexane, 2,2-dimethylbutane,
and 2-methylpentane (102). Another early example is the removal of tetrahydrothiophene (THT)
from methane by means of a fixed bed containing
Cu3(BTC)2 (3). The color of the Cu3(BTC)2
changed from deep blue to light green, indicating
that THT was adsorbed to the copper open metal
sites. More recently, it has been reported that Mg
and Fe-MOF-74, packed in a column, exhibit enhanced carbon dioxide separation from methane
and C1 to C3 hydrocarbon, respectively (61, 103).
Proton Conductivity for Fuel Cell Applications
The use of MOFs as inexpensive proton-conducting
membranes for fuel cell applications represents a
new direction in MOF research (104). The first
conductivity measurements were performed on
Cu(DTOA)(HOC2H4)2, albeit with a relatively low
reported conductivity (105). It is now understood
that any improvement in the water-mediated proton
conductivity of MOFs requires the addition of an
acid functionality, such as carboxylic, phosphonic,
or sulfonic acid, as demonstrated by several functionalized versions of MIL-53 [Fe(OH)(BDC-X);
X = H, NH2, OH, (COOH)2] (106). This finding is
further supported by the demonstrated high proton
conductivity—2.5 × 10−3 S/cm at 98% relative
humidity and 60°C—of MOFs with highly acidic
pores [PCMOF-5, La(H5DTP)(H2O)3; DTP8– =
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1,2,4,5-tetrakisphosphonomethylbenzene] (107).
Alternatively, high proton conductivity is also observed in one-dimensional (1D) metal-organic
structures lacking any acidic functionality, as in
Fe(oxalate)(H2O)2. Here, coordinated water molecules act as proton donors, with their 1D arrangement within crystals facilitating the rate of proton
transport (108).
If MOFs are to serve as proton conductors for
practical applications, they must be able to function at relatively high operating temperatures (120°
to 180°C) and in anhydrous conditions (109). A
simple strategy that may be used to meet these
practical considerations is the impregnation of MOFs
with amphoteric molecules such as imidazole.
For instance, 1H-1,2,4-triazole–loaded PCMOF-2
[Na3(THBTS); THBTS3– = 2,4,6-trihydroxy-1,3,5benzenetrisulfonate] has a higher conductivity than
the MOF without triazole (110). Similar results were
also obtained for imidazole-doped Al(OH)(NDC)
(NDC = 1,4-naphthalenedicarboxylate) (111). Even
higher conductivity was observed when histamine
was used as a proton carrier (112). These exciting
results indicate that high-mobility proton carriers
situated within the pore play a key role in achieving MOFs with high proton conductivity.
Extensive research has been devoted to realizing polymer-supported electrolyte membrane
fuel cells consisting of perfluorosulfonic acid
polymers (e.g., Nafion); however, these polymers
have some drawbacks, such as operation temperature, humidity, and cost (109). MOFs will be
attractive candidates for this application because
of their tunable pore size and functionality as well
as their chemical and thermal stability.
MOF Nanocrystals
The advantages of MOFs are not solely limited to
properties that originate from their pore structure
and functionalities. By virtue of the diverse synthetic procedures reported, the size and morphology of MOF nanocrystals can be precisely controlled
(113). Several groups have recently reviewed the
preparation of MOF nanocrystals for membrane
and thin-film applications (2, 114, 115).
In 2003, the synthesis of nanocrystalline MOFs
was accomplished at room temperature for
MOF-5, affording crystal sizes of 70 to 90 nm
(116). The obtained MOF samples were crystalline and porous but without controlled morphology. It was not until 2009 that precise control of
MOF nanocrystal morphology was reported (117):
rhombic dodecahedral ZIF-8 nanocrystals with
an estimated average diameter of 40 nm, narrow
size distribution, high crystallinity, porosity, and
chemical stability. A follow-up study revealed that
the nucleation process in ZIF nanocrystal formation is initially slow, followed by a rapid process
of crystal growth (118). Interestingly, the study
showed that the morphology of the nanocrystals
transformed from cubic to rhombic dodecahedra in the last stage of crystal formation. This
result indicated that the morphology of nanocrystals can be tuned through the optimization
of synthetic conditions. Furthermore, soc-MOF-1
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{[M6O2(ADB)3(H2O)6](H2O)6(NO3)2; ADB4– =
3,3′,5,5′-azobenzenetetracarboxylate; M = In(III),
Ga(III)} and HKUST-1 nanocrystals have adopted
several morphologies with their respective diameters in the submicrometer to micrometer
range (119, 120).
The assembly of MOF nanocrystals is of particular interest in MOF research because organized nanostructures comprising one or more
nanocrystal types may have distinctive properties
similar to those seen in metal nanocrystal superlattices (121). Only a limited number of examples
have been reported to date; however, closely packed
2D superlattice structures of ZIF-8, soc-MOF-1,
and UiO-66 were observed by SEM (119, 122, 123).
Applying an ac electric field created linear chain
structures of ZIF-8, and the alignment of the nanocrystals was controlled by the anisotropic nature
of surface (124). Clearly, the synthesis of nanocrystals, their assembly into larger structures, and
their incorporation into devices are research areas
of increasing interest.
The Materials Beyond
At present, MOF chemistry has matured to the point
where the composition, structure, functionality,
porosity, and metrics of a metal-organic structure
can be designed for a specific application. This
precise control over the assembly of materials is
expected to propel this field further into new
realms of synthetic chemistry in which far more
sophisticated materials may be accessed. For example, materials can be envisaged as having (i)
compartments linked together to operate separately, yet function synergistically; (ii) dexterity
to carry out parallel operations; (iii) ability to count,
sort, and code information (7); and (iv) capability
of dynamics with high fidelity (125, 126).
To appreciate how such materials may be
achieved, it is important to step back and consider
the gamut of materials studied thus far. In general, crystalline materials are largely composed of
a few building units as constituents of their underlying chemical structure. For example, common
solid-state materials such as zeolites, alloys, polymers, and even MOFs and nanocrystals are each
constructed from one or two kinds of repeating units
(Fig. 5). The same is true for the entire landscape
of human-made crystalline materials. This selfsimilarity of structures is quite unlike the heterogeneity and multiplicity that characterize biological
molecules. However, the building of materials from
as few as three or four different building units remains an outstanding challenge because such chemistry is believed to give mixed phases rather than a
single phase comprising mixed components.
It is our view that vast opportunities exist for
making materials from an increasing number of
unique building units and that a transformative
chemistry is yet to be uncovered for achieving
these materials. This new materials space that is
expected to emerge from linking multiple kinds
of building units will have unlimited possibilities,
especially when the building units are arranged in
specific sequences within crystals. We propose to
1230444-8
call this space “the materials beyond.” One can
imagine materials with unique sequences of building units that code for specific functions. Materials
capable of complex operations and transformations could be used in applications such as conversion of carbon dioxide to fuels, methane to
higher alkanes, and water to hydrogen. Once we
begin to think in terms of heterogeneity of composition, shape, size, and orientation of building
units, there is no limit to the different scenarios
for making materials endowed with unusual complexity. Such materials go beyond merely linking
building units through strong bonds, as in MOFs.
In 2010, a simple strategy was proposed and
demonstrated to introduce further complexity to
crystalline structures (Fig. 6A) (7). In this work,
18 MTV-MOF-5–type structures were synthesized
using variously functionalized BDC2– linkers
[NH2, Br, (Cl)2, NO2, (CH3)2, C4H4, (OC3H5)2,
and (OC7H7)2]. One of these structures contains
eight different functionalities in one phase. The
backbone structure (same as MOF-5) is ordered,
whereas functional groups incorporated into the
framework are varied and disordered. The complex arrangements of these functionalities in the
pores can lead to unusual properties; for instance,
MTV-MOFs showed improved hydrogen and
carbon dioxide uptake capacities relative to MOFs
comprising a single organic linker (7). This finding clearly demonstrates that MTV-MOFs are not
simple linear combinations of their constituents.
Precise control of the functionality arrangement
in MOFs is still a looming challenge; however, a
new strategy that is widely used for peptide synthesis has been used to systematically control a sequence of functionalities via solid-phase synthesis.
Metal-organic complex arrays (MOCAs) are heterometallic complexes in which different metalorganic centers are connected to an amino acid
backbone (Fig. 6B) (127). The solid-phase synthesis strategy for connecting these units allowed
for careful control of the sequence, nuclearity, and
length of up to six metal centers including Pt(II),
Rh(III), and Ru(II). Potentially MOCAs can serve
as functionalities within MOFs to affect sequencedependent properties. This idea may also manifest
itself in materials recently prepared by the sequential deposition of multilayer MOF thin films (128).
It is not always necessary to exercise multiple
linkers or specific sequences to acquire heterogeneity in MOFs without losing either crystallinity or porosity. Recently, microporous MOF-5
materials in which each crystal contains a system
of mesopores and macropores have been synthesized by use of a controlled amount of 4(dodecyloxy)benzoic acid (DBA) to a reaction
Self-similarity
1
2
Complexity
Building blocks
3
4
5
Materials Beyond
Metals, alloys, silicon, metal oxides,
zeolites, polymers, MOFs, nanocrystals, ...
Proteins,
RNA, DNA, ...
Fig. 5. Crystalline chemical structures of some of the most useful materials. These materials
(metals, alloys, silicon, metal oxides, zeolites, and polymers) are constructed from few kinds of building
units (one to three) to make largely self-similar materials in which such building units repeat periodically
throughout the material. Few examples exist of synthetic crystals with more than three building units, as
found in biological structures (proteins, RNA, and DNA). The linking of building units by strong bonds
(reticular chemistry) as exemplified by the construction of MOFs is useful in developing the structure and
function space of the “materials beyond.”
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www.sciencemag.org
REVIEW
REVIEW
A
D
Zn4O(CO2)6
COOH
X
COOH
MTV-MOF-5
), Br (
), NH2 (
), NO2 (
), C4H4 (
), (CH3)2 (
), (OC3H5)2 (
), (OC7H7)2 (
MOF-1001
)
B
PQT
Metal chelating unit
2+
PQT
2+
X=H(
Metallation
Amino acid backbone
= Pt(II),
= Rh(III),
= Ru(II)
PQT2+/MOF-1001
Resin
2XN
N N
Ru2+
N N
N
Cl
Cl
Rh
N N
N
Cl
O
H2N
N
O H
O H
N
N
N
O H
O H
N
O
N
N
X-
Pt+
Cl
Cl
N
Cl
Rh
N
N
O
O
Cleavage
Cl
N
N
O H
O H
N
Fmoc
O
N
N
X-
Pt+
Cl
O
N
N
N
Pt+ XCl
Metal-organic complex array
(MOCA)
C
Zn4O(CO2)6
pmg-MOF-5
COOH
OC12H25
+
COOH
COOH
Fig. 6. Examples of creating heterogeneity within crystalline materials. (A) MTV-MOF-5, in which a heterogeneous mixture of functionalities
decorates the interior of the crystals to provide an environment capable of
highly selective binding of carbon dioxide. (B) Schematic representation of the
synthesis of MOCAs, which have a heterogeneity created by arranging metal
complexes in specific sequences to give sequence-dependent properties. (C)
www.sciencemag.org
SCIENCE
Heterogeneity within order produced in crystals of MOFs (pmg-MOF-5), where
mesopores and macropores are encased in microporous MOF-5 to give unusual carbon dioxide capture properties. (D) A MOF (MOF-1001) constructed
from linkers having polyether rings, which in themselves create stereoelectronically specific binding of substrate paraquat (PQT2+). Atom colors are as in
Fig. 2; blue polyhedra, Zn. Hydrogen atoms are omitted for clarity.
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30 AUGUST 2013
1230444-9
mixture of MOF-5 (Fig. 6C) (129). By addition
of DBA, an entirely sponge-like MOF-5 crystal
was obtained, whereas mesopores and macropores fully enclosed by a thick microporous MOF5 sheath (pomegranate-like crystals; pmg-MOF-5)
were obtained when a lesser amount of DBA was
used. In situ synchrotron powder x-ray diffraction
measurements revealed that the mesopores and
macropores of pmg-MOF-5 can act as additional
carbon dioxide adsorption sites, even though the
sponge-like MOF-5 crystals did not show such
behavior.
Heterogeneity can also be introduced when
multiple metal ions are used in the synthesis of
materials termed core-shell structures (130). Unlike
metal-doped MOFs (131), core-shell structures form
two compositionally distinct solid phases at their
respective interface. Although little is known about
the interface of the two phases, film structural analysis of single crystals containing both Zn and Cu domains {Zn2(NDC)2(DABCO)/Cu2(NDC)2(DABCO);
DABCO = 1,4-diazabicyclo[2.2.2]octane} reveals that the Cu phase was grown on the surface
of the Zn phase with a small in-plane rotation angle (130). Precise control of such angles may create
interfaces that can be exploited for separation and
filtration applications.
MOFs comprising interconnected architectural domains that operate independently are also of
high interest. In 2009, a “concept transfer” from
the biological world successfully accomplished
this goal (126). MOF-1001 [Zn4O(BPP34C10DA)3;
BPP34C10DA2- = 4,4′-(2,5,8,11,14,16,19,22,25,28decaoxa-1,15(1,4)-dibenzenacyclooctacosaphane12,15-diylbis(ethyne-2,1-diyl))dibenzoate] uses
periodic crown ether receptors attached to the architectural framework; this endows the pore with
active domains capable of molecular recognition of highly disordered guests in a stereoelectronically controlled fashion (Fig. 6D) (125).
This advancement exploited the mechanism of
conformational changes that afford biological macromolecules the capabilities of molecular recognition and installed a new active domain that
functions independently of the traditional sorting
(i.e., shape and size selectivity of the pore aperture) and coverage domains (i.e., internal pore surface binding). Further evidence for the potential
creation of such materials was reported in a MOF
[Zn(Gly-Ala)2; Gly-Ala = glycylalanine] constructed
from a flexible dipeptide linker (132). This MOF
displayed adaptable porosity as a result of the
linker, evoking comparisons to the conformational
selection that is characteristic of proteins.
MOF architectures reminiscent of Russian
dolls composed of nested interconnected
cages {cage-within-cage structure; CPM-7
[Zn 26 O 3 (OH) 4 (FDA) 30 ·(H 2 O) 12 (Et 2 NH 2 ) 6 ]
(FDA2– = furan-2,5-dicarboxylate), CPM-24
[Co9(OH)2(acetate)(BTC)4(IN)8·(H2O)4(Me2NH2)5]
(IN– = isonicotinate)} can be another way to
introduce irregularity within a single parent structure (133, 134). These nested structures provide
exciting potential for the assembly of various
connected domains whose internal and external
1230444-10
environments can be manipulated to operate independently to suit particular functions (133, 134).
The chemistry and applications of MOFs
have progressed substantially since their original
inception more than a decade ago. Although it is
difficult to rule out the possibility of major advancements arising from MOFs that are compiled from a limited number of building blocks, it
is our belief that the future of MOFs lies in the
creation of materials whose constituents are many
and are systematically varied.
18.
19.
20.
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Acknowledgments: We acknowledge the original
contributions made by members of the Yaghi research
group as cited in the references. In particular, preliminary
work on this manuscript was done by W. Morris, F. Gándara,
H. Deng, and Y. Zhang. Supported by BASF SE (Ludwigshafen,
Germany); U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, Energy Frontier Research
Center grant DE-SC0001015; U.S. Department of Defense,
Defense Threat Reduction Agency grant HDTRA 1-12-1-0053;
and the Ministry of Education, Science and Technology
(Korea), WCU Program grant NRF R-31-2008-000-10055-0.
Supplementary Materials
www.sciencemag.org/cgi/content/full/341/6149/1230444/DC1
Materials and Methods
Figs. S1 to S8
Tables S1 to S5
References (135–363)
Related Web sites
CSD reference codes for MOFs
10.1126/science.1230444
www.sciencemag.org
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Metal-Organic Frameworks - Omar Yaghi`s Laboratory

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